Various analytical procedures to detect an analyte in a test sample are known in the prior art.
For example, immunoassays use the mechanisms of the immune system, wherein antibodies and the respective antigens are capable of binding to each other.
This specific reaction mechanism is used to determine the presence or quantity of the antigen in a test sample. In particular, the antibody or the antigen (analyte of interest) is labeled to quantify the interactions between antibody and antigen. Common labels are, for example, fluorescent and chemiluminescent molecules, colored particles (beads) or radioisotopes.
Recently, magnetic labels have been used in microfluidic assays to detect the presence or quantity of an analyte. The use of magnetic labels as, for example, magnetic particles, also denominated as magnetic beads or beads, has several advantages. The magnetic particles can be actuated by applying a magnetic field such that the analytical procedure can be accelerated. Further, there is no magnetic background signal in the biological test sample influencing the detection of the magnetic particles.
However, these assays using magnetic labels require means for actuating the magnetic particles bound to the antigens to be immobilized near the sensor surface of the sensor cartridge, and for flushing away the remaining unbound magnetic particles not to influence the quantity measurement of the bound particles. Therefore, for example, two magnets may be arranged on opposite sides of the sensor cartridge, wherein the first magnet attracts the magnetic particles to move through the test sample toward the sensor surface, then the second magnet attracts unbound magnetic particles to move away from the sensor surface. In this configuration, the two magnets are mounted on a holding structure, and the holding structure mechanically moves the magnets toward or away from the sensor surface (see R. Luxton et al., “Use of External Magnetic Fields to reduce reaction times in an immunoassay . . . ”, Anal. Chem. 2004, 76, 1715-1719).
Such a method is very laborious and time-consuming and needs a complex holding structure for arranging the two magnets on opposite sides of the sensor cartridge. Further, the first magnet arranged below the sensor cartridge controls the movements of the magnetic particles only in the direction perpendicular to the sensor surface, but not in the horizontal direction, the direction essentially parallel to the sensor surface. Therefore, areas with accumulations of unbound magnetic particles may exist in the cartridge next to areas with only few or maybe too less magnetic particles to bind with the antigens of interest. Further, unbound particles in peripheral regions in the cartridge may not easily and as fast as other particles be attracted by the second magnet so that these particles may remain in the cartridge. This may result in unreliable test results.
Generally, the particles of a test sample undergo several processes, e.g. particles approach to the sensor surface, bind to the sensor surface, unbind from the sensor surface, etc. In known biosensor systems, magnetic particles near by the magnet are usually actuated by the magnetic field and drawn towards the magnet. In this case, the quality and/or quantity of a signal received from the sensor surface next to the magnet will depend on time and thus not be reliable, since it would not only represent the bound particles, but also be influenced by unbound magnetic particles in remote sections in the cartridge which are actuated by the magnetic field and may thus move towards the sensor surface next to the magnet.